Low power circuits are widely used in mobile devices. To meet the low power requirement, subthreshold leakage (dependent on Vt (voltage threshold)), gate leakage and gate induced drain leakage need to be controlled very precisely. In the low power circuit, both the low Vt device and the high Vt device have different circuit performance requirements, and the Vt difference between high Vt and low Vt can be up to 200 mV.
As the channel length and gate oxide are scaled down, higher halo or channel doping are needed to maintain high Vt (˜0.5V) for low power applications. However, the high doping increases the junction and gate induced drain leakage (GIDL), which is not acceptable for low power devices which have Vt around 0.5V and Ioff around 0.01 nA/um.
Gate materials with different work function are used to adjust Vt as shown in equation 1, below.Vt=φm−φs−Qox/Cox+2ψB+(4εsiqNaψB)1/2/Cox   (1)φm and φs are work functions of the material forming the gate and of the silicon substrate. Qox represents the charge in the gate dielectric, and Cox is the gate dielectric capacitance. ψB is the difference of potential between Fermi's level and the intrinsic Fermi level. Na is the doping concentration in the channel.
By way of example, fully silicide (FUSI) gate devices have attracted much attention because it can reduce the gate depletion and improve the device performance. In such an application, midgap workfunction gate material is applied to both high Vt and low Vt devices. But, the silicide has a workfunction close to the midgap of Si which requires count doping for low Vt (˜200 mV) devices. This is because positive φm−φs is required for nFET devices and negative φm−φs is required for pFET devices.
But, the count doping will degrade the Vt-roll off and performance. Also, midgap workfunction gate material is very good for high Vt devices because high Vt can be achieved with low channel/halo doping. Therefore, the junction and GIDL leakage can be reduced which is critical for low power application because the total Ioff leakage is very small. But, this process is not beneficial for low Vt devices, since the Vt in such devices would be too high.